DIFFERENTIAL CALCIUM SENSITIVITY OF 2 TYPES OF QUANTUM BUMPS IN LIMULUS VENTRAL NERVE PHOTORECEPTORS

Ventral photoreceptors in Limulus polyphemus were continuously illumin ated with dim light and produced two types of quantum bumps. Bumps of one type had a relatively symmetrical shape and fast rise and decay ki netics. In addition, bumps were detected with a small amplitude and sl ower kinetics. We termed these bump types ''C-2'' and ''C-1 bumps'', r espectively. The half width of a bump divided by its amplitude was fou nd to be a reliable criterion to distinguish between the bump types. T his shape quotient is smaller than 0.5 ms/pA for the C-2 bumps and lar ger than 1 ms/pA for the C-1 bumps. Lowering the extracellular calcium concentration from 10 mM to 0.25 mM caused an increase in the average amplitude of C-2 bumps to 196%. After the injection of small amounts of 1.2 mM EGTA solution the amplitude of these bumps was reduced to ab out 30 and 50% in two cells studied. By contrast, in both cases C-1 bu mp amplitudes were not affected significantly. We conclude that the tw o bump types are generated in one photoreceptor by the activation of d ifferent transduction pathways. These pathways are differentially sens itive to changes of the cytosolic calcium concentration.